In horizontal directional drilling (HDD), boreholes are drilled into the earth in generally horizontal directions. A drill string is provided with a bit body, which couples to a drill bit or a blade. The drill bit (or blade) and drill string drill the borehole.
Such boreholes are used, for example by utilities for communication lines, sewer lines, etc. If a utility line is to be buried beneath a road, rather than tear up the road and disrupt traffic, a borehole is drilled beneath the road and the line is pulled through.
In HDD, it is desirable to control the direction of the borehole. This allows the borehole to be drilled at a controlled depth and miss other buried items such as lines, building foundations, tunnels, etc. A transmitter, or sonde, is provided in the drill string close to the drill bit. The sonde is located in a housing that forms part of the drill string. The housing has a cavity for the sonde. Ports or slots extend from the cavity to the outside of the housing. These slots allow the transmitter signal to escape the metal housing. Without the slots, then the housing typically provides too much shielding and attenuates the transmitter signal to the point where it cannot be detected on the surface.
On the surface, an operator uses a receiver to determine the location of the sonde and thus the borehole. In this manner, the borehole can be drilled and extended to the desired locations.
Currently, the slots in the sonde housing are open to the exterior. Debris enters the cavity through the slots. The debris packs in around the sonde, making removal of the sonde difficult. The sonde must be removed periodically to replace its batteries.
In the prior art, some efforts have been made to close off the slots by epoxy. In Blair, U.S. Pat. No. 6,349,778, the slots for the transmitter signal are filled with epoxy. In addition, the sonde is located on a polyurethane liner. However, during drilling operations, the rotating drill sting encounters severe vibrations as the drill bit cuts its way through the earth. These vibrations shake loose such materials used to fill up the slots. Also, the epoxy in the slots can be pushed into the sonde housing and damage the sonde.
The sonde housing is typically located near the bit body. Various types of drill bits and blades are used in drilling. For example, a tooth bit is used for hard conditions where rock is encountered. The bit has teeth projecting therefrom, which teeth contact the rock. More moderate or soft conditions may not need a tooth bit, but rather a blade. Such moderate or soft conditions have little or no rock. It is desirable to easily configure the drill string to match the conditions encountered in the drill hole.
Different bit bodies are used to match the bit or blade needed. For example, a specific bit body is used with a drill bit, while another bit body is used with a blade. Drill bits encounter high loading (such as side loads) during drilling operations. The bit body is matched to the drill bit in order to accommodate the loads. Such bit bodies cannot be used with blades.
In addition, tooth-type drill bits have drill teeth that project forward from the drill bit. Different drilling conditions may call for different tooth arrangements. Yet in the prior art, in order to vary the tooth arrangement, another drill bit must be used.